International Journal of Electrical, Electronics and Data Communication, ISSN: 2320-2084
Volume-3, Issue-6, June-2015
RESEARCH ON CONTROL STRATEGY OF HYBRID ACTIVE
POWER FILTER
1
1,2,3
XU YAN, 2LU GUANGXIANG, 3YI GUIPING
School Of Electrical Engineering, Southeast University, Nanjing 210096, China
Abstract- The construction and working principle of shunt active power filter are introduced, current hysteresis tracking
control method, triangular wave modulation control method are discussed in detail. Experimental simulation is carried out in
MATLAB environment and its results show that this kind of HAPF has excellent filtering effect under the action of the
control strategy.
Key words- hybrid active power filter; MATLAB; control strategy; simulation
of current tracking control circuit, drive circuit and
main circuit). The core of command current
arithmetic circuit is to detect harmonic and reactive
current components in compensation object current,
so it is sometimes also called harmonic and reactive
detection circuit. The purpose of compensation
current generating circuit is to generate actual
compensation current according to the command
signals of compensation current generated from
command current arithmetic circuit. The main circuit
currently used is PWM converter [2].
I. INTRODUCTION
With the rapid development of modern industrial
technology and power electronics technology, the
application of different kinds of inverters, converters,
switches and reactors keeps increasing together with
the harmonic pollution arising therefrom, resulting in
serious distortion of grid voltage, affecting the normal
operation of instruments, aggravating the wear and
tear to power components and imperiling the safe
operation of power system. Currently, harmonic has
become one of the serious pollutions of power
system, so it is particularly urgent to solve harmonic
problems in power system. Apart from the traditional
methods of using passive filters and changing grid
structures, active power filters (APF) are effective
devices for treating harmonic. In order to improve
APF performance, it is vital to study APF control
strategies [1,2].
The basic working principle of APF is: detect the
voltage and current of compensation object, and
calculate the command signal of compensation
current through command current arithmetic circuit.
This signal will be magnified through compensation
current generating circuit to generate compensation
current, which is offset by the harmonic and reactive
currents, etc. to be compensated in load current, thus
getting the expected power current. In Figure 1, set
the load current as iL, and the harmonic iLh detected
by the command current arithmetic circuit as the
command current
ic* of APF; the output
compensation current ic of compensation current
generating circuit will follow the change to the
command current ic*, and ic and iLh, will offset
each other. Then grid current is=iL-ic=iLf, i.e.,
equal to the fundamental wave current of load, so that
power current becomes sinusoidal wave. If reactive
current of load also needs to be compensated, only
the component with reverse polarity of load current
fundamental wave reactive component needs to be
added in compensation current command signal. In
this way, the harmonic and reactive components of
power current and load current will offset each other,
and the power current equals to the fundamental
wave active component of load current. Similarly,
APF may also compensate the negative-sequence
current of asymmetrical three-phase circuit [2-4].
II. WORKING PRINCIPLE OF APF
Figure 1 Schematic Diagram of APF
Figure 1 is the schematic diagram of the most
fundamental APF system components, with the load
as the harmonic source (i.e., compensation object)
and APF as the active power filter. According to
Figure 1, APF system is composed of two major
parts, i.e., command current arithmetic circuit and
compensation current generating circuit (consisting
For voltage-type APF, the main circuit uses a
voltage-type converter. The main circuit of paralleltype three-phase three-wire system APF is shown in
Figure 2 [5].
Research On Control Strategy Of Hybrid Active Power Filter
68
International Journal of Electrical, Electronics and Data Communication, ISSN: 2320-2084
Volume-3, Issue-6, June-2015
cause the relative following error of compensation
current to be too large with small ic; on the other
hand, fixed hysteresis width may also cause the
switching frequency of devices to be too high with
large ic, which may even exceed the allowable
maximum operating frequency of devices, thus
resulting in device damage. Large range of switching
frequency change indicates that hysteresis control not
only has the characteristic of pulse width modulation,
but also contains the component of pulse width
modulation. It is the combination of the two
modulations. For the control over power converter,
frequency modulation is usually unwanted. In the
hysteresis instantaneous comparison control method,
the hysteresis width of hysteresis comparator has a
relatively large impact on the following performance
of compensation current. When the hysteresis width
is relatively large, the switching frequency of power
semiconductor switching device is relatively low,
thus having low requirement on switching devices;
however, the following error is relatively large and
the higher harmonic in compensation current is
relatively large. On the contrary, when the hysteresis
width is relatively small, though the following error is
relatively small, the switching frequency is relatively
high. Therefore, when the hysteresis comparison
control method is adopted, appropriate hysteresis
width should be set and
variable hysteresis width should be adopted if
necessary, so as to achieve better tracking effect. This
control method is featured by relatively simple
hardware circuit, which is usually constructed with
simulation circuit; since the method belongs to realtime control method, the current response is quick;
since carrier wave is unnecessary, the output voltage
contains no harmonic components of specific
frequency.
Figure 2 Main Circuit of Three-phase Three-wire System APF
III. APF CONTROL STRATEGIES
The purpose of compensation current control circuit
is to get the PWM signals controlling the connection
of each device of compensation current generating
circuit according to the interrelation between
compensation current command signal and actual
compensation current, and the control result should
follow in real time the changes to the command
signal. In order to guarantee good real-time of
compensation current, the current control should use
tracking-type PWM control method, which currently
mainly include hysteresis comparison, triangular
wave comparison and dead-beat control [5-7].
A. Hysteresis comparison
Figure 3 is the schematic circuit diagram using
hysteresis comparison for control. In this formula, the
command signal ic* and the compensation current
signal ic actually sent by the inverter are compared,
and the deviation of the two is i . As the input of
hysteresis comparator, it will generate the PWM
signal controlling the connection of switching devices
in main circuit, and this PWM signal will, after
magnification through drive circuit, control the
connection of the power devices of main circuit, thus
controlling the change to compensation current ic.
The output of hysteresis comparator is a series of
pulse sequences with relatively large width and
irregular distribution, and the hysteresis width has a
relatively large impact on pulse width [8].
B. Triangular wave comparison
Figure 4 Schematic Diagram of Triangular Carrier Wave
Modulation
Figure 4 is the schematic diagram of triangular carrier
wave modulation control method, which is different
from other PWM control methods using triangular
wave as carrier wave in that it compares the deviation
c i
between ic* and ic after amplifier A with
triangular wave rather than compares command
signal ic* with triangular wave directly. Amplifier A
is usually a proportional amplifier or a proportionalintegral amplifier. Such control system is designed
based on the minimization of c i
control.
Compared with the instantaneous value comparison
method, the triangular carrier wave modulation
control method has the following characteristics: the
hardware is relatively complicated and it is usually
Figure 3 Instantaneous Value Comparison Method of
Hysteresis Comparator
It is beneficial to use this control method for APF due
to the advantages of hysteresis control such as quick
response speed, easy realization and unnecessary
understanding of load nature (inductive load or
capacitive load). However, this hysteresis control
method has a large range of switching frequency
change. Particularly, when c i change has a large
range, on the one hand, fixed hysteresis width may
Research On Control Strategy Of Hybrid Active Power Filter
69
International Journal of Electrical, Electronics and Data Communication, ISSN: 2320-2084
realized by adopting the digital control method with
DSP as the core; it has a relatively large following
error compared with the hysteresis comparison
method; the switch frequency of devices is fixed and
equals to the frequency of triangular carrier wave; the
current response is slower than that of the
instantaneous value comparison method [8,10].
Volume-3, Issue-6, June-2015
the voltage of capacitance at converter DC side
should be controlled at a suitable value. The control
method of DC side voltage Uc is shown in
Figure 5.
2.3 Phase shift SPWM control of combined
converter
In order to realize effective compensation to reactive
current and higher harmonic current, switching
devices are required to work under higher frequency.
However, the switching frequency of high-power
sinusoidal pulse width modulation (SPWM) converter
will be limited due to the following two reasons: (1)
the switching
frequency of high-power
semiconductor devices (e.g., GTO) is relatively low;
and (2) high switching frequency will result in
relative large switching loss, thus reducing system
efficiency. However, multiplexed power converters
have relatively poor regulation performance, which
could not fully meet the requirement of modern grid.
Therefore, Document [9] proposes the phase shift
SPWM technology of combined converter. The
essence of this technology is the organic combination
of multiplexed technology and PWM technology,
which may realize the phase shift SPWM technology
of high-power converter under low switching
frequency, significantly reduce the harmonic output
and improve the output waveform, thus reducing the
capacity of wave filters. Meanwhile, phase shift
SPWM converter has better dynamic response and
relatively higher transmission band, enabling the
application of many cutting-edge control means and
improving control performance. Current-type
converter has the advantages such as direct power
supply, reliable operation and simple protection, and
is applicable in many high-power applications. For
example, grid active compensation devices, if
adopting current-type phase shift SPWM technology,
may realize the advantages such as simple structure,
good control performance, quick response, broad
frequency band and strong harmonic elimination
capacity. This technology solves the contradiction
between high power equipment and low switchingfrequency devices, and enables the use of converters
composed of super high power devices such as GTO
in APF equipment. Therefore, this technology has
broad applications in high-power applications such as
APF [9].
Figure 5 Command Current Operation Containing DC Side
Voltage Control
According to the Figure, the difference between the
given value and the feedback value of DC side
voltage will, after PI regulator treatment, generate the
regulation signal ip, which will be added to overlay
the DC component of the instantaneous active
current; after operation, the command signal will
contain certain fundamental wave active current, and
the compensation current generating circuit will
generate compensation current according to the
command signal and input it into grid, so that the
compensation current in APF will have certain
fundamental wave active current component, thus
enabling energy exchange between DC side and AC
side of APF and regulating the DC side voltage to the
given value [10,11].
V.SIMULATION STUDY
Current tracking control circuit is the primary link of
compensation current generating circuit, and its
purpose is to get the PWM signals controlling the
connection of each device of compensation current
generating circuit according to the interrelation
between compensation current command signal and
actual compensation current, so that the
compensation current will track the change to the
command signal.
A. Hysteresis control
Comprehensive comparison analysis on APF could be
performed though the model established, with nonlinear load adopting thyristor three-phase bridge
fully-controlled rectifier circuit
The detailed parameters are as follows:
IV. VOLTAGE CONTROL AT DC SIDE OF
MAIN CIRCUIT
As a major part of hybrid APF, compensation current
generating circuit is composed of voltage-type PWM
converter and other corresponding drive circuits and
current tracking control circuits. In order to guarantee
good compensation current following performance,
Research On Control Strategy Of Hybrid Active Power Filter
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International Journal of Electrical, Electronics and Data Communication, ISSN: 2320-2084
Volume-3, Issue-6, June-2015
TABLE 1 Comparison of Grid Side Current Harmonic
Content before and after Compensation When the firing angle
is 30°
Figure 6 Simulation Model of Hysteresis Control Circuit
Figure 7 A-phase Current Waveform before Compensation
Figure 11 A-phase Current Waveform before Compensation
Figure 8 A-phase Current Waveform after Compensation
Figure 12 A-phase Current Waveform after Compensation
Figure 9 A-phase Current Spectrogram before Compensation
Figure 13 A-phase Current Spectrogram before Compensation
Figure 14 A-phase Current Spectrogram after Compensation
Figure 10 A-phase Current Spectrogram after Compensation
Research On Control Strategy Of Hybrid Active Power Filter
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International Journal of Electrical, Electronics and Data Communication, ISSN: 2320-2084
Volume-3, Issue-6, June-2015
TABLE 2 Comparison of Grid Side Current Harmonic
Content before and after Compensation When the firing angle
is 45°
TABLE 3 Comparison of Grid Side Current Harmonic
Content before and after Compensation When the firing angle
is 60°
Figure 15 A-phase Current Waveform before Compensation
From Figure 7 to Figure 18 is the analysis of a-phase
current waveform and spectrum before and after APF
compensation. Obviously, with the increase of firing
angle, the distortion of current waveform becomes
more serious. From Table 1 to Table 3, the current
before compensation has the highest 5th harmonic
content, and the content of major harmonics
decreases with the increase of harmonic number.
When APF is input, the content of 5th, 7th and 11th
major harmonics decreases significantly, indicating
that the compensation purpose of APF is effective
and significant.
Table 4 Distortion Rate Corresponding to Different Thyristor
Firing Angles
Figure 16 A-phase Current Waveform after Compensation
According to Table 4, when the thyristor firing angle
increases gradually from 30°, the current distortion
rate will also increase gradually before and after
compensation.
Therefore,
the
compensation
performance of APF will decrease with the increase
of thyristor firing angle.
B. Triangular wave control
Triangular wave control model has basically identical
parameters with hysteresis control model.
The details are as follows:
Figure 17 A-phase Current Spectrogram before Compensation
Figure 18 A-phase Current Spectrogram after Compensation
Research On Control Strategy Of Hybrid Active Power Filter
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International Journal of Electrical, Electronics and Data Communication, ISSN: 2320-2084
Volume-3, Issue-6, June-2015
CONCLUSION
This paper analyzes the APF control strategies,
including PWM control methods and inverter DC
side voltage control principles. PWM control
methods mainly include hysteresis comparison,
triangular wave comparison and dead-beat control.
This Chapter is the simulation comparison analysis
on the first two control methods. The experimentation
shows that both the methods may effectively
restraint harmonic, but the hysteresis control has
better tracking performance under same conditions.
Figure 19 Circuit Simulation Model of Triangular Wave
Modulation Control Method
REFERENCES
[1]
Xiao Xiangning. Analysis and Control of Power quality[M].
Beijing: China Electric Power Press, 2004.
[2] M. EI-Habrouk, M. K. Darwish, P. Mehta. A Survey of
Active Filters and Reactive Power Compensation
Techniques.
Power Electr.
And V.S.D. Conference
Publication. September 2000:712.
[3] FU Lan-fang,SUN He-xu,DONG Yan. Sample Time
Staggered One-cycle Control Strategy of Three-phase
Cascade
Inverter[J].
Industry
and
Mine
Automation,2012,38(9):51-54.
[4] Liu Hankui, Gu Jianjun, Xu Dianguo, Wang Yan. Parallel
Active Power Filter Using Hysteresis Current Controller[J].
ELECTRIC DRIVE，2001, (5):34-37.
[5] Wu Hongyang, He Xiangning. Research on PWM Control of
A Cascade Multilevel Converter[J]. PIEMC, 2000, 3:10991103.
[6] LI Hong-yu, ZHUO Fang, LEI Wan-jun, et al. Control
system of the multiple large power rate active power filter[J].
Advanced Technology of Electrical Engineering and
Energy,2004,23(1):25-28
[7] Corasaniti V F, Barbieri M B, Arnera P L, et al. Hybrid
active filter for reactive and harmonic compensation in a
distribution network[J]. IEEE Trans. Ind. Electron,
2009,56(3):670-677.
[8] YI Guiping, HU Renjie. Research on Simulation of
Modeling and Simulation Analysis of Hybrid Active
Power Filter Based on PSCAD/EMTDC[J]. Power &
Energy，2014,35(3):275-278.
[9] Zhang Zhongchao,Ooi B T. Multi-modular Current Source
SPWM Converter for SMES[J]. IEEE Trans.
PE,1993,8(3):250-256.
[10] Xu Wanfang, Luo An, Wang Lina, Tu Chunming.
Development of Hybrid Active Power Filter Using
Intelligent Controller[J]. Automation of
Electric Power
Systems,2003,27(10):49-52.
[11] HOU Chong, XIAO Tie-jun, FAN Yan, LI Ping. Research
on Shunt Active Power Filter with Matlab
Simulation[J]. Power System and Clean Energy,
2013,29(9):11-17.
[12] Du Guanjun,Liu Xinyang,Wang Jie. Analysis and Simulation
of Current Tracking Control Methods for
Hybrid Active Power Filter Device[J]. Electric Power
Science and Engineering, 2011,27(6):6-10.
Figure 20 A-phase Current Waveform after Triangular Wave
Modulation
Figure 21 A-phase Current Spectrogram after Triangular
Wave Modulation
Figure 21 is the a-phase current spectrogram after
triangular wave modulation of APF under the
triangular wave control method. According to this
Figure, the content of 5th, 7th, 11th and 13th
harmonic is 1.15%, 1.68%, 1.94% and 1.36%
respectively, while the total distortion rate of a-phase
current before compensation is 27.48%. However, the
total distortion rate of a-phase current after
compensation under the hysteresis control method is
5.01%. Therefore, with same compensation object,
hysteresis control method has better compensation
effect tan triangular wave control method.
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